1. Technical Field of the Invention
The present invention generally relates to the art of electrical current sources/sinks and, more particularly, to a current source with improved output impedance using bipolar transistors.
2. Description of Related Art
Many circuits require current sources/sinks that are relatively constant and highly insensitive to changes in supply. In addition, a most desirable characteristic of current source/sink circuits is a high output resistance or impedance. This improves the accuracy for the output signal and results in a high voltage gain for the circuit if the circuit is used as an active load in an amplifier for example. When using PNP/NPN bipolar transistors cascode current sources/sinks, two problems in particular arise, base currents and base current modulation, contribute to undesirable variations to the output. Base currents, and hence base current errors, result from finite transistor Beta. In contrast, finite Early voltage and/or changes in the output voltage lead to base current modulation errors. The above-described errors within the cascode transistor limit the output impedance of the final circuit. It would be desirable to provide a current source/sink circuit that removes these limitations.